The Buddha is The Awakened One. It is what we aspire to be: awakened, enlightened to our true nature.
The root word budh means to open your eyes, or 'wake up.' There are many Buddhas. The 'historical' Buddha is Buddha Shakyamuni from the Shakya clan of India who lived around 2,500 years ago when he purified all of the defilements of his mind. He then realized during meditation (under the famous bodhi tree) the skilful means of love, compassion and wisdom, and developed one of the most sophisticated, sacred philosophies and traditions ever recorded. Today of course this is called Buddhism.
The Dharma is the complete body of teachings that Buddha gave. It is the essential philosophy that all may attain peace and joy because fundamentally that is our true nature.
Imagine The Buddha as an amazingly effective, intuitive doctor of complementary and alternative medicine. The Dharma is the perfect medicine that cures root causes, and The Sangha is like a loving supportive nurse who assists us in taking our medicine.
The Sangha is the community of ordained monks, nuns and lay persons that practice the Dharma. The Buddha taught that we should surround ourselves with those of similar interests, teachers and lay practitioners to inspire us in the beneficial activities of the Dharma.